The preparation of the mixtures of methylene-bridged polyphenyl polyamines (hereafter polymethylene polyphenyl polyamines) containing di(aminophenyl)methanes by condensation of aniline and formaldehyde under aqueous conditions in the presence of strong acids, particularly hydrochloric acid, has been widely described. Illustrative of such processes are those shown in, for example, U.S. Pat. Nos. 2,638,730; 2,950,263; 3,260,751; 3,277,173; 3,297,759 and 3,476,806. Such processes, in one form or another, are widely used commercially to prepare the polyamines in question, which polyamines are employed as intermediates in the preparation of the corresponding isocyanates, i.e. diisocyanatodiphenylmethane and mixtures of polymethylene polyphenyl polyisocyanates. The isocyanates are employed in, the preparation of a variety of polyurethanes, polyisocyanurates and other polymers (both cellular and non-cellular), which can be derived from polyisocyanates.
For many purposes, it is desirable that the diamine content of the above polyamines, and the diisocyanate content of the polyisocyanates derived therefrom, contain a high proportion of 4,4′-isomer. This has been achieved largely by employing proportions of strong acid in the condensation of the aniline and formaldehyde with the aniline present in an amount of at least 2 mole per mole of formaldehyde.
There are a number of reasons why it is undesirable to use these high concentrations of strong acid in the condensation. Thus, the presence of the strong acid, particularly in the later stages of the condensation, which are conducted at elevated temperature, represents a serious corrosion problem involving constant repair and maintenance requirements that contribute significantly to the overhead cost of operation of the manufacturing plants in which such processes are utilized. Alternatively, more expensive equipment may be used. Further, the acid in question has to be neutralized, usually by means of aqueous sodium hydroxide, at the end of the reaction and disposal of the resulting neutral salt solution represents a severe problem because of the vast volumes of such material that are generated.
Processes have been described that eliminate the use of the strong acid catalysts and the necessity to neutralize the reaction products and substitute solid catalysts such as clay, zeolites and diatomaceous earth (see, for example, U.S. Pat. Nos. 3,362,979; 4,039,580; and 4,039,581). However, these processes give rise to products in which the 4,4′-isomer content of the diamine is substantially reduced in favor of the 2,4′-isomer, and, in some cases, the 2,2′-isomer.
We have now surprisingly found that the condensation of aniline with formaldehyde, and the conversion of the condensation product to the desired polyamines, can be effected using specific heterogeneous catalysts without having the above mentioned drawbacks.